Revert pwsh preference for Windows upgrades

[svarlamov]

Motivation

• Simplify Windows upgrade behavior by reverting the pwsh-first spawn logic because the PowerShell checksum fallback implemented in install.ps1 removes the need to prefer pwsh.

Description

• Remove the spawn_powershell closure that tried pwsh then powershell and instead invoke powershell directly via Command::new("powershell") while keeping -NoProfile, -ExecutionPolicy Bypass, -Command, CREATE_NO_WINDOW, the GIT_AI_RELEASE_ENV env var, and the silent stdout/stderr behavior.

Testing

• No automated tests were run as part of this change (CI/integration validation is expected to run in workflows).

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Codex Task

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 4 additional findings in Devin Review.

[devin-ai-integration]

🟡 Accessing potentially uninitialized $sha256 in finally block throws under strict mode

In the SHA256 fallback path of Verify-Checksum, if [System.Security.Cryptography.SHA256]::Create() on line 83 throws before assigning to $sha256, the finally block at line 88 references $sha256 which was never set. Because Set-StrictMode -Version Latest is active (install.ps1:2), accessing an uninitialized variable throws a RuntimeException, masking the original error.

Root Cause

PowerShell try/finally blocks share the same scope, so $sha256 is visible in finally only if the assignment on line 83 succeeded. If SHA256::Create() throws, $sha256 is never assigned. Under strict mode, if ($sha256) on line 88 raises:

The variable '$sha256' cannot be retrieved because it has not been set.

This replaces the original exception, making it harder to diagnose the real failure. The fix is to initialize $sha256 = $null before the try block.

Impact: On the rare occasion that SHA256::Create() fails (e.g., FIPS policy restrictions), the user sees a confusing "variable not set" error instead of the actual SHA256 creation failure.

Suggested change

	if ($sha256) {
	$sha256.Dispose()
	}
	if ($null -ne $sha256) {
	$sha256.Dispose()
	}

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